Physiological Basis of Sleep and Wakefulness

Introduction

Knowledge about the physiological basis of sleep has increased rapidly over the last few years. It is now recognized to be a highly complex and heterogeneous state, which is intimately connected with the state of wakefulness. There is a dynamic balance between the processes controlling both these states, which is complex and has important implications for many sleep disorders.

Functions of sleep
The functions of sleep are still uncertain, but NREM and REM sleep almost certainly have different functions. Sleep is in many ways a vulnerable state, because of the reduced awareness and responsiveness to the environment, but it has been highly conserved during evolution suggesting that it has a survival advantage. The proposed functions for sleep fall into the following categories.

Biochemical
Several anabolic hormones, such as growth hormone, are secreted primarily during sleep, whereas catabolic hormones, such as cortisol, are produced mainly during wakefulness. The metabolic rate slows during NREM sleep, in which energy is conserved, the body temperature falls, and protein synthesis and other anabolic processes are accentuated.

Wakefulness may also have important biochemical effects on neurones which are compensated for by sleep. During wakefulness neuronal glucose utilization is more rapid, and intracellular stores of glycogen within astrocytes are consumed. This process is reversed during sleep so that glucose can be available for neuronal activity and to enable normal metabolic functioning during the next episode of wakefulness.

Replenishment of other metabolites or removal of oxygen free radicals during NREM sleep may also be important.

Physiological
Sleep has been considered to be a restorative or a recovery phase, or to prepare the organism physiologically for the next phase of wakefulness. Cell division is most rapid during NREM sleep, at which time protein synthesis is increased. Sleep also has important effects on the immune system and is itself influenced by, for instance, cytokines which are an integral component of immunity. Energy is conserved during sleep, but this is only between 100 and 200 calories per night and is probably of little significance.

Neurological
Synchronization of cortical activity during NREM sleep may in some way coordinate cortical networks. The prefrontal cortex is inactive during NREM sleep as well as REM sleep, and this may also have some benefit.

REM sleep may have a neurodevelopmental role. It is most prolonged in mammals whose offspring are least mature at birth, and in neonates and young children. The ability to form new neurones (neurogenesis) slows early in life and new behaviour patterns are mainly due to the development of neural networks.

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During REM sleep the cerebral cortex is open to sensory inputs and makes loose associations which are not possible during wakefulness. The basal ganglia are also active so that behavioural patterns that are essential for survival can be developed without them being manifested by motor activity. The REM sleep processing and integration of newly acquired information into existing neural templates enables future responses to also reflect the previous experience of the individual and the inherited potential [1]. These are given an emotional charge through the activity of the limbic system.

Psychological
Both NREM and REM sleep appear to be involved in consolidation of memory, but they almost certainly have different influences on this. Acquisition of new information during sleep is extremely limited, but consolidation or maintenance of memory from experiences during the previous day is considerable [2]. Learning of visually acquired information is improvedduring the first night of sleep, and sleep deprivation on this night impairs recall of information. Retention is best if stages 3 and 4 NREM sleep in the first 2h of the night are followed by REM sleep in the last 25% of the night. The sequence of NREM and REM sleep appears to be important.

Learning of motor sequences improves during the first night of sleep and appears to be dependent particularly on stage 2 NREM sleep later in the night, during which individual components of the learnt sequence can be integrated, particularly the most complex parts.

Recall of cognitive procedures is better if there is a sequence of NREM and then REM sleep, but declarative memory appears not to require REM sleep [3].

Probabilistic learning, in which associations are made according to the likelihood of events being related, improves after sleep on the first night after the experience. Declarative memory during sleep may be related to spindle activity in stage 2 NREM sleep [4].

Dreams are a manifestation of the underlying cerebral activity and reflect the loose mental associations of REM sleep which enable new neuronal networks to be formed, probably promoting creative mental activity and improving problem-solving ability. At sleep onset explicit images of the day's events are often recalled if the subject is awoken during stage 1 NREM sleep, but as the REM sleep episodes progress during the night these become incorporated into associative networks and are less readily recognizable.

This creative activity of REM sleep complements its function in memory consolidation.

Social
In non-developed societies it is usual for some members of a group to be awake while others are sleeping in order to afford protection for the group. The rapid reversibility of sleep to the waking state in response to significant stimuli also gives protection against adverse environmental events.

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